1. Field of the Invention
The present invention relates generally to a swirl type diesel engine and more specifically to such a diesel engine wherein the crown of the piston is formed with a two stage flame splitting/swirl inducing arrangement which improves dispersion and mixing of flame which is ejected from the swirl chamber.
2. Description of the Prior Art
FIG. 1 shows a so called "clover leaf" type flame dispersing cavity or recess arrangement 1 which is used in combination with a swirl or prechamber type diesel engine of the nature disclosed in JP-A-54-59512, JM-A-50-138403 and JM-A-57-78724.
With this type of engine, in order to lower the peak combustion temperature in the swirl chamber 2 and thus reduce the amount of NOx produced, a fraction of the fuel which is injected thereinto is released into the main combustion chamber prior spontaneous combustion of the main air-fuel charge in the swirl chamber.
However, in the absence of any strong turbulence or other mixing influence, most of the fuel which is permitted to escape into the main combustion chamber via a transfer passage 3 tends to either remain in, or close to the trench portion 4 of the cavity. As the cavity 1 exerts only a relatively weak dispersive influence on the flame which is ejected into the trench 4, it tends to flow up and over the rounded nose portion 5 of the clover leaf type dispersion recess located opposite the trench. This induces the situation wherein the fuel droplets in the trench tend to be entrained in a flame plume FP having a shape essentially as illustrated in broken line.
Under these conditions, the fuel which is released into the main combustion chamber tends to be carried by the initial flame ejection into a relatively cool section thereof wherein quenching of the flame is apt occur.
Accordingly, the relatively rich flame and the entrained fuel (particularly the latter) are not mixed effectively with the air which is available in the main combustion chamber and neither tend to undergo sufficiently rapid nor complete combustion to enable good engine output and low noxious emissions.
As a result of this problem, during low load engine operation high HC emissions result while under higher loads smoke formation tends to exceed acceptable limits.